Field of the Invention
The present invention relates to a damping force variable shock absorber, and more particularly, to a damping force variable valve assembly having a low speed control valve capable of controlling an extremely low speed flow rate and a low speed flow rate when a damping force variable valve operates in a soft mode.
Description of the Related Art
In general, a shock absorber is installed in means of transportation, such as automobile, and improves a ride comfort by absorbing and damping a vibration or shock from a road surface during driving.
The shock absorber includes a piston rod installed in a cylinder to perform compression and rebound strokes, and a piston valve disposed in the cylinder in a state of being connected to the piston rod to generate a damping force.
When the damping force is set to be low, the shock absorber can improve ride comfort by absorbing a vibration caused by unevenness of a road surface. On the contrary, when the damping force is set to be high, the shock absorber can improve steering stability by suppressing a change in a posture of a vehicle body. Therefore, in the past, a damping force variable shock absorber, to which a damping force characteristic is differently set according to the purpose of use of the vehicle, has been applied to a vehicle.
Recently, there has been developed a damping force variable shock absorber in which a damping force variable valve capable of appropriately adjusting a damping force characteristic is mounted on one side, such that the damping force characteristic is appropriately adjusted to improve a ride comfort or a steering ability according to a road surface and a driving condition.
FIG. 1 is a sectional view illustrating an example of a damping force variable shock absorber according to the related art. Referring to FIG. 1, the damping force variable shock absorber 10 according to the related art includes a base shell 12, and an inner tube 14 which is installed inside the base shell 12 and in which a piston rod 24 is movably installed in a length direction. A rod guide 26 and a body valve 27 are installed in an upper portion and a lower portion of the inner tube 14 and the base shell 12, respectively. In the inside of the inner tube 14, a piston valve 25 is connected to one end of the piston rod 24, and the piston valve 25 partitions the internal space of the inner tube 14 into a rebound chamber 20 and a compression chamber 22. A top cap 28 and a base cap 29 are installed in an upper portion and a lower portion of the base shell 12, respectively.
A reservoir chamber 30 is formed between the inner tube 14 and the base shell 12 to compensate for a change in the volume of the inner tube 14 according to the reciprocating motion of the piston rod 24. A flow of a working fluid between the reservoir chamber 30 and the compression chamber 22 is controlled by the body valve 27.
In addition, a separator tube 16 is installed inside the base shell 12. Due to the separator tube 16, the inside of the base shell 12 is partitioned into a high pressure chamber PH connected to the rebound chamber 20, and a low pressure chamber PL serving as the reservoir chamber 30.
The high pressure chamber PH is connected to the rebound chamber 20 through an inner hole 14a of the inner tube 14. The low pressure chamber PL is connected to the compression chamber 22 through a lower passage 32 formed between a body portion of the body valve 27 and the base shell 12 (or the base cap 29) and a passage formed in the body valve 27.
Meanwhile, the shock absorber 10 according to the related art includes a damping force variable valve assembly 40 mounted on one side of the base shell 12 so as to vary a damping force.
The damping force variable valve assembly 40 is provided with oil passages respectively connected to the base shell 12 and the separator tube 16 and communicating with the high pressure chamber PH and the low pressure chamber PL. In addition, the damping force variable valve assembly 40 includes a spool 44 installed to be moved by a driving of a plunger 42. An inner passage communicating with the high pressure chamber PH and the low pressure chamber PL is varied by the movement of the spool 44, and the damping force of the shock absorber 10 is varied accordingly. The plunger 42 is configured to move in a horizontal direction, when viewed in FIG. 1, due to a magnetic force generated when an electric current flows through a solenoid.
In the damping force variable valve assembly according to the related art, for example, when the plunger 42 moves to one side (left side in FIG. 1), the spool 44 closes a passage to generate a high damping force (hard mode). On the contrary, when the plunger 42 moves to the other side (right side in FIG. 1), the spool 44 opens a passage to generate a low damping force (soft mode).
In the technical field to which the invention pertains, many efforts have been made to provide a shock absorber having a good damping force variable characteristic by improving the performance of a damping force variable valve assembly. For example, Korean Patent Application Publication Nos. 10-2010-0023074 and 10-2010-0007187 disclose techniques of damping force variable valve assemblies recently developed for a shock absorber.
A damping force variable valve assembly according to the related art controls a final exit by using a disk valve in a hard mode and discharges a fluid through a slit formed in the disk valve so as to control a damping force with respect to an extremely low flow rate and a low speed flow rate.
In a case where the damping force variable shock absorber according to the related art operates in the hard mode, when a moving speed of a working fluid is high, that is, when a flow rate is high, a resistance is determined by a sagging amount of main disk valves stacked in a valve assembly.
However, in a case where an input (that is, shock) from a road surface is small and thus a moving speed of a working fluid is low or extremely low, a flow rate of the working fluid flowing in the shock absorber is extremely low. As a result, a resistance caused by a slit (that is, a fixing orifice) is almost not formed. Thus, a damping force is not generated in a low speed section and a small frequent vibration transferred from the road surface cannot be damped.
In order to solve this problem, a size of a slit may be reduced to increase a damping force at a low speed in a hard mode. However, in this case, due to characteristics of an orifice, a damping force is rapidly increased as a flow rate is increased.
In addition, when the size of the slit is reduced, the size of the fixing orifice is reduced. Accordingly, a hard damping force or dispersion may greatly occur and foreign substances may clog the slit, thus degrading operation performance.